The present invention relates to a positioning control method and apparatus for use in a movement apparatus for effecting positioning with high accuracy for a large amount of movement.
FIGS. 8, 9 and 10 are block diagrams of a conventional positioning control method of a positioning apparatus for a large amount of movement with high accuracy in combination with a coarse movement mechanism and a fine movement mechanism.
In the conventional positioning control method of FIG. 8, a coarse movement mechanism 1 is moved with respect to an apparatus and a fine movement mechanism 2 is moved with respect to the coarse movement mechanism 1.
The coarse movement mechanism 1 has low positioning accuracy but can be moved in a wide area, while the fine movement mechanism 2 can be moved in a narrow area with high positioning accuracy.
When it is assumed that an absolute displacement of a reference point of the coarse movement mechanism 1 with respect to a fixed point of the apparatus is a displacement F and a relative displacement of the fine movement mechanism 2 with respect to the reference point of the coarse movement mechanism 1 is a displacement G, a displacement H of an object on the fine movement mechanism 2 with respect to the fixed point of the apparatus is a sum of G and F.
A signal generator 4 constituted by a computer or the like prepares a movement command A for designating a position of the object on the fine movement mechanism 2 with respect to the fixed point of the apparatus.
The displacement H of the object on the fine movement apparatus 2 with respect to the fixed point of the mechanism is detected as a displacement B by position detecting means 5 such as, for example, a laser measuring device or the like.
A difference signal C which is equal to a value obtained by subtracting the displacement B from the movement command A is supplied to the coarse movement mechanism 1 and the fine movement mechanism 2 through input switching means 3.
In brief, in the positioning control method shown in FIG. 8, the movement command A is prepared by the signal generator 4 constituted by a computer or a signal generator. The absolute displacement H of the object to be positioned with respect to the fixed position which is a sum of the relative displacement G of the fine movement mechanism 2 with respect to the coarse movement mechanism 1 and the absolute displacement F of the coarse movement mechanism 1 with respect to the fixed point is detected as the displacement B by means of the position detecting means such as, for example, a laser measuring device or the like. The difference signal C between the movement command A and the displacement B is supplied to the input switching means 3 constituted by a computer or an analog circuit, for example.
When the difference signal C is larger as compared with a movable area of the fine movement mechanism 2, the input switching means is connected to a and a coarse movement command signal D is supplied to the coarse movement mechanism 1. The coarse movement mechanism 1 is controlled to be moved within the movable area of the fine movement mechanism by means of the difference signal C. When the difference signal C falls within the movable area of the fine movement mechanism 2, the input switching means 3 is connected to b and a fine movement command signal E is supplied to the fine movement mechanism 2. By moving the fine movement mechanism 2 so that the difference signal C is substantially equal to zero, the absolute displacement H of the object to be positioned with respect to the fixed position is caused to coincide with the movement command signal A.
A conventional positioning control method shown in FIG. 9 controls a coarse movement mechanism and a fine movement mechanism having the same configuration as that of the positioning control method of FIG. 8.
When it is assumed that an absolute displacement of a reference point of a coarse movement mechanism 11 with respect to a fixed point of an apparatus is a displacement F1 and a relative displacement of a reference of a fine movement mechanism 12 with respect to the reference point of the coarse movement mechanism 11 is a displacement G1, a displacement H1 of an object on the fine movement mechanism 12 with respect to the fixed point of the apparatus is a sum of G1 and F1.
A signal generator 14 constituted by a computer or the like prepares a movement command A1 for designating a position of the object on the fine movement mechanism 12 with respect to the fixed point of the apparatus.
A displacement H1 of the object on the fine movement mechanism 12 with respect to the fixed point of the apparatus is detected as a displacement B1 by means of position detecting means 15 such as, for example, a laser measuring device or the like.
An error signal C1 equal to a value obtained by subtracting the displacement B1 from the movement command A1 is supplied to the coarse movement mechanism 11 and the fine movement mechanism 12 through a compensation circuit 16, a limiter circuit 17 and a insensitive band circuit 18.
In brief, in the positioning control method shown in FIG. 9, the movement command signal A1 is prepared by the signal generator 14. The absolute displacement H1 of the object to be positioned with respect to the fixed position which is equal to a sum of the absolute displacement G1 of the fine movement mechanism 12 with respect to the coarse movement mechanism 11 and the absolute displacement F1 of the coarse movement mechanism 11 with respect to the fixed position is detected as the displacement B1 by means of the position detecting means 15. The difference signal C1 between the movement command A1 and the displacement B1 is supplied to the compensation circuit 16 for compensating a gain characteristic and a phase characteristic of the object to be positioned to thereby obtain a compensation error signal J1. The compensation error signal J1 is supplied to the coarse movement mechanism 11 and the fine movement mechanism 12 simultaneously. The compensation error signal J1 supplied to the fine movement mechanism 12 passes through the limiter 17 and the compensation error signal J1 supplied to the coarse movement mechanism 11 passes through the insensitive band circuit 18.
The fine movement mechanism 12 responds to a specific error area of -.alpha. to .alpha. within the movable area of the fine movement mechanism 12 and the coarse movement mechanism 11 responds to the outside of the error area or the external area larger than .alpha. and smaller than -.alpha.. The limiter 17 produces an output signal proportional to an input signal when the input signal is within the specific error area and produces an output signal having a fixed value when the input signal is outside of the specific command area. The insensitive circuit 18 does not produce any output signal when an input signal is smaller than a specific value command .alpha. or greater than-.alpha. and produces an output signal proportional to a difference of the input signal and the specific value command .alpha. when the input signal is not smaller than the specific value command .alpha., and produces an output signal proportional to the difference of the input signal and the specific value-.alpha. when the input signal is not greater than the specific value-.alpha..
When the command signal C1 is outside of the specific command area, the coarse movement mechanism 11 is moved a long distance, and when the command signal C1 comes within the specific command area, the fine movement mechanisms 12 is moved so that the signal C1 is substantially equal to zero. Accordingly, the absolute displacement H1 of the object to be positioned with respect to the fixed position substantially coincides with the movement command signal A1 to effect the positioning (refer to Nakagawa, Torii and Kubota; "Development of Combined Fine and Coarse Movement Super-Precision Servo Technique--Trial Manufacture and Estimation", 1990, Collected Treatises of Lectures of Autumn Scientific Lecture Meeting of The Japan Society for Precision Engineering, pp. 1055).
In a conventional control of FIG. 10, a coarse movement mechanism 21 is moved with respect to a fixed point of an apparatus and a fine movement mechanism 22 is moved with respect to the coarse movement mechanism 21.
The fine movement mechanism 22 and the coarse movement mechanism 21 include measuring devices 25a and 25b for measuring displacement with respect to the fixed point of the apparatus, respectively.
In the positioning control method shown in FIG. 10, a signal generator 24 supplies to the fine movement mechanism 22 a fine movement command signal A3 having a fine movement step hm as a resolution as shown in FIG. 11 as a movement command signal. Further, the signal generator 24 supplies to the coarse movement mechanism 21 a coarse movement command signal A2 for generating a coarse movement step hc (=3.times.hm) as a movement command signal as shown in FIG. 12 each time three fine movement steps hm of FIG. 11 are generated.
The coarse movement command signal A2 to the coarse movement mechanism 21 is not produced until time t1 that the fine movement steps hm of the fine movement command signal A3 are generated three times and the coarse movement mechanism is not moved. On the contrary, the fine movement mechanism 22 is moved in response to the fine movement command signal A3 as follows. An error signal C3 obtained by subtracting a displacement signal B3 produced from the position detecting means 25a for detecting an absolute displacement value H2 with respect to the fixed point of the apparatus of the object on the fine movement mechanism 22 from the fine movement command signal A3 is supplied to a compensation circuit 29 constituted by a computer or an analog circuit for compensating a gain characteristic and a phase characteristic of the object to be positioned. The fine movement mechanism 22 receives fine movement compensation command signal E2 produced by the compensation circuit 29 and is moved to be substantially coincident with the fine movement command signal A3.
When the fine movement steps hm for the fine movement command signal A3 are generated three times and the time t1 comes, the coarse movement command signal A2 generates a coarse movement step corresponding to three time the fine movement steps hm. An error signal C2 obtained by subtracting from the coarse movement command signal A2 a displacement signal B2 produced by the position detecting means 25b for detecting the absolute displacement value of the coarse movement mechanism with respect to the fixed position is supplied to the compensation circuit C2. The coarse movement mechanism 21 receives a coarse displacement compensation command signal D2 and is moved to be coincident with the coarse movement command signal A2.
On the other hand, the fine movement mechanism 22 continues the same movement as that made until the time t1. Accordingly, movement value G2 of the fine movement mechanism at the time t1 is G2=3 hm, and movement value F2 of the coarse movement mechanism is F2=hc=3 hm.
Accordingly, the absolute displacement value H2 of the object to be positioned with respect to the fixed position is H2=F2+G2=6hm. At this time, the displacement signal B3 produced by the position detecting means 25 is subtracted from the fine movement command signal A3. Accordingly, the fine movement mechanism 22 is supplied with error signal C3=A3-B3=3 hm-6 hm=-F3 and is moved back by the movement value F2 of the coarse movement mechanism.
The positioning control method shown in FIG. 10 performs the above operations repeatedly to perform positioning over a long distance with high accuracy (refer to Sakuta, Ogawa and Ueda; "Study Concerning Super-Precision Position Control-Interlocked Control of Coarse and Fine Movement", Collected Treatises of Lectures of Spring Scientific Lecture Meeting of The Japan Society for Precision Engineering, 1990, pp. 795, and Sakuta, Ogawa and Ueda; "Position Control by Interlocked Coarse and Fine Movement", Collected Treatises of Lectures of Spring Scientific Lecture Meeting of The Japan Society for Precision Engineering, 1991, pp. 229).
In order to perform continuous control in which an object to be positioned traces the locus to be targeted with high accuracy over a long distance, a positioning apparatus including a coarse movement mechanism having a large amount of movement and low positioning accuracy and a fine movement mechanism having a small amount of movement and high positioning accuracy and on which an object to be positioned is mounted is employed. In this case, a movement command having accuracy corresponding to that of the fine movement mechanism is inputted.
Since the input to the fine movement mechanism 2 and the coarse movement mechanism 1 is switched by the input switching means 3 in accordance with a magnitude of the error signal C in the controller shown in FIG. 8, the absolute displacement value H of the object to be positioned with the fixed position is substantially coincident with the movement command signal A when the fine movement mechanism 2 is within the movable area. However, when the fine movement mechanism 22 is moved to the movable area, the error signal C is increased gradually since the fine movement mechanism 2 can not be moved. When the error signal C reaches a magnitude in which the input switching means 3 is switched to the coarse movement mechanism 1, the coarse movement mechanism 1 is moved in response to the error signal C.
Accordingly, in the continuous positioning control, since the fine movement mechanism 2 can not be moved and only the coarse movement mechanism 1 is moved in accordance with the above procedure, the continous positioning control with high accuracy is not attained.
Further, in the positioning control method of FIG. 9, the compensation command signal J1 obtained by supplying the command signal C1 to the compensation circuit 16 is selectively supplied to the limiter 17 or the insensitive band circuit 18 in accordance with the magnitude of the compensation command signal J1 to produce the fine movement compensation command signal E1 or the coarse movement compensation command signal D1.
At this time, as described above, when the continuous positioning control with high accuracy is made, since the movement command signal A1 is inputted with accuracy corresponding to that of the fine movement mechanism 12, the fine movement mechanism 12 is moved to the movable area of the fine movement mechanism 12 and the absolute displacement amount H1 of the object to be positioned is substantially coincident with the movement command signal A1. However, when the movement command signal A1 is a command signal so that the movable area of the fine movement mechanism 12 is exceeded, the fine movement mechanism 12 can not be moved although the compensation command signal J1 is supplied to the fine movement mechanism 12. Thus, since the command signal C1 is increased gradually, the compensation command signal J1 is also increased. When the compensation difference signal J1 exceeds the difference area of -.alpha. to .alpha. set by the limiter 17 and the insensitive band circuit 18, the compensation command signal J1 is supplied to the coarse movement mechanism 11 and the coarse movement mechanism is moved.
Thus, since the fine movement mechanism 12 already exceeds the movable area, the fine movement mechanism 12 can not be moved for the continuous positioning control and only the coarse movement mechanism 11 is moved, so that the continuous positioning control with high accuracy is not attained.
Further, in the positioning control method of FIG. 10, when the continuous positioning control with high accuracy is made, the fine movement command A3 having the fine movement step hm corresponding to the accuracy of the fine movement mechanism 22 is supplied to the fine movement mechanism 22 as shown in FIG. 11. The coarse movement step hc having a resolution or more with which the coarse movement mechanism 21 can be moved and having an integral multiple, for example three times of the fine movement step hm is supplied to the coarse movement mechanism 21 as shown in FIG. 12 at the same time as the time t1 or t2 that the fine movement command signal A3 generates the fine movement steps hm three times.
At this time, as described above, the fine movement mechanism 22 is moved back by the movement amount F2 of the coarse movement mechanism to attain positioning.
Since the coarse movement mechanism 21 generally has low positioning accuracy, a positioning error necessarily occurs in the coarse movement mechanism movement amount F2 for the coarse movement command signal A2. Accordingly, since the fine movement mechanism 22 is moved to correct the positioning error, the positioning error of the coarse movement mechanism is accumulated for each repetition of the above control method. Thus, the movable area of the fine movement mechanism 22 is exceeded and the fine movement mechanism can not be moved. Accordingly, the continuous positioning control with high accuracy can be not be made thereafter.
As described above, even in any positioning central method shown in FIGS. 8 to 10, when the object to be positioned is positioned with high accuracy over a long distance, the fine movement mechanism necessarily exceeds the movable area and can not be moved, so that the positioning with high accuracy can not be made for the continuous movement command signal.
Further, even in any apparatus, it is impossible to control the continuous positioning in a large amount of movement with high accuracy and movement of a point to a point that high-speed movement is made between two points of a starting point of the object to be positioned and a final positioning point to effect positioning to the final positioning point by means of the same control method, and accordingly control methods must be selectively used in accordance with movement to be made.